Ammonia dissociator



Dec- 11, 195 w. H. MARSHALL, JR

AMMONIA DISSOCIATOR Filed March 1, 1949 INVENTOR ll/lll VA WALTON H. MARSHALL JR.

BY Swww ATTO NEYS Patented Dec. 11, 1951 AMMONIA DISSOCIATOR Walton H. Marshall, J r., New York, N. Y.

Application March 1, 1949, Serial No. 79,087

1 Claim.

, This invention relates to the dissociation of ammonia gas into a mixture of nitrogen and hydrogen. More specifically, it relates to an improved apparatus for this purpose, which is compact, easy to handle, and entirely safe for use in the home, particularly by children.

It is well known that ammonia gas, when passed at low pressure over a suitable catalyst, such as reduced iron oxide, at elevated temperatures, breaks down almost completely into nitrogen and hydrogen according to the chemical equation 2NH3- 3H2-I-N2. Commercial apparatus for producing ammonia dissociation gas is standard equipment for industrial uses. Ammonia dissociators are employed in industry for many purposes, such as producing hydrogen for welding, and generating a reducing gas for atmosphere control in metallurgical processing.

These ammonia dissociators are cumbersome pieces of equipment, entirely unsuited for use in the home and unsafe except when operated by skilled personnel, since ammonia gas released into a confined space such as a room can be both explosive and toxic. For example, a standard ammonia dissociator capable of producing 150 cubic feet per hour of dissociated gas weighs about one ton, exclusive of ammonia storage bottles, and is equipped with industrial control instruments such as temperature controllers, high and low temperature alarms, pressure regulators and flow-meters. Obviously this equipment is too large, expensive, cumbersome and unsafe for home use.

The principal object of the present invention resides in the provision of an improved apparatus for dissociation of ammonia, which is of a compact construction adapted for manufacture at low cost, and can be operated by unskilled persons without danger of forming a toxic or harmful explosive mixture. Thus, the apparatus can be operated, for example, by unattended small children to produce gas for the purpose of filling balloons.

' For a better understanding of the invention, reference may be had to the accompanying drawing, in which the single illustration is a vertical sectional view of a preferred form of the new apparatus.

1 The apparatus as illustrated comprises a pressure containerl for storing liquid ammonia, the container having an outlet valve 2 which is adjustable manually to control the rate of the ammonia fed from the container. The container also has a restricted discharge orifice 3 through which the ammonia gas expands as it discharges from the container. A special union or connector 4 i disposed between the restriction orifice 3 and a duct 5, the union 4 comprising two telescoping sections 4a and 4b. A ball valve 40 is normally hel-d seated against a restricted outlet 4d in the section do by the ammonia gas pressure in the container, and a device ie on the section 4b is operable to unseat the ball valve when the union sections are telescoped together to make the connection. The union 4 thus prevents escape of ammonia gas from the container except when the sections 4a and 4b are connected together for operation of the apparatus.

The duct 5 is coiled around an insulating body in the form of a vertical core 6 mounted on a base I. In the upper part 6a of the core is a chamber 8 containing a catalyst, such a reduced iron oxide, adapted to dissociate the ammonia gas into a mixture of nitrogen and hydrogen at elevated temperatures. The coil or duct 5 communicates at its upper end with the lower portion of the catalyst chamber 8. At its upper portion, the chamber 8 is provided with an outlet 8a for dissociated ammonia, and this outlet is normally closed by a valve 9. A thermostat 9a, such as a bimetallic strip, is connected to valve 9 and is operable to open the valve only when the chamber 8 is heated to a temperature at which substantially all of the ammonia gas fed into the chamber is dissociated, as will be pointed out in greater detail presently.

A hollow member II in the form of a cylinder is mounted on the base .I and surrounds the core 6 in spaced relation thereto. The surrounding member II extends upwardly from the base beyond the catalyst chamber 8 and forms with the core 6 an annular passage or space I2 communieating with the outlet 8a. The dissociated ammonia discharged through the thermostatic valve 9-911 flows downwardly through the passage I2 and finally discharges through a conduit I3 communicating with the lower portion of the passage I2. The conduit I3 may be provided with a suitable fitting I3a which may be inserted int the neck of a balloon to be inflated.

The hollow member II is provided at its upper end with a cover I4 for sealing the discharge passage I2. On the lowerface of the cover I4 is a block I5 made of a heat insulating layer I5 which overlies the chamber 8 and forms with the insulating body or core part 611 a radial passage I2a which serves as a communication between outlet 8a and the annular passage I2. Electrical conductors I6 extend downwardly through the insulating'layer I5 andradial passage IZa to an electrical heating element [1 coiled around the chamber 8 within the insulating body 6a. The conductors l6 extend through openings in the cover [4 which are sealed, as shown at Mia, and the outer ends of the conductors are connected to an electrical plug l8 adapted to be inserted into an electrical supply outlet. The coil l1 and its conductors serve as a means for heating the chamber 8 to a temperature, for example, 1200" F., at which the ammonia gas will be dissociated in the presence of the catalyst. A switch 19 i inserted between the coil I1 and its plug I8 and may be provided with a timer mechanism to limit the period of operation of the'apparatus.

The hollow member ll includesa layer of insulating material 2| in the form of a jacket surrounding the catalyst chamber 8. Thejacketextends downwardly from a flange on the cover l4 and terminates at its lower end a substantial distance below the chamber 8. The part of the member Ii between the base 1 and the insulating jacket 2| consists only of metal or other material which is a good conductor of heat, for a purpose to be described presently.

-An outer insulatin jacket 22 is mounted on the base i and forms with the hollow member I l a vertical passage 23 for a cooling medium. As shown, the jacket 22 has openings 22a in its lower portion, and a top provided with openings 22b. The openings 22a and 22b serve-as inlets and outlets, respectively, for air which is circulated through passage 23 as the cooling medium. The lower openings 22a also serve for passages of the duct and the conduit l3, while the upper openings 22b serve for passage of the conductors I6.

In the following description of the operation of the-apparatus, it will be assumed for illustrative purposes that the pressure container l for liquid ammonia has a capacity of approximately .5 1b.-of liquid ammonia and has .4 square foot of external surface, and that the flow restricting orifice 3 limits the flow of ammonia vapor to 12 cubic feet per hour. These limitations are desirable where the apparatus is to be used in the home for inflating balloons, or the like, as will be-described more fully presently.

When the connection 4 is made, ammonia gas flows from the container I through the valve ie-4d and the duct 5 leading to the converter,

where it flows upwardlythrough the heat ,exchange coil formed by duct 5 and into the catalyst or reaction chamber 8. However, "the thermostatic valves-9a prevents escape of gas from the catalyst chamber until the temperature ,level in the chamber has reached a point where ammonia is substantially completely decomposed by heat and the catalyst. Accordingly, in order to obtain discharge of gas from the apparatus, it is necessary first to connect the plug 18 to an electrical outlet and turn on switch l9, thereby enervalve therefore prevents discharge of gas at any temperature below that at which ammonia is substantially completely decomposed by the catalyst. As shown, the outlet 8a in the valve seat is conical and the valve 9 is of similar form, so that as the temperature increases above the level at which the thermostat opens the valve, the thermostat will displace the valve farther from its seat and thereby increase the throughfiow area of the outlet 8a, permitting a greater flow of dissociated gas. The higher gas flow rate tends to cool the reaction or catalyst zone, and eventually a steady condition is reached. Accordingly, the thermostatic valve serves the dual purpose of preventing outward flow of any substantial quantity of undissociated ammonia and of preventing the catalyst zone from overheating to the point where the apparatus might be damaged.

The valve 9 could, of-course, be located in the inlet line to the catalyst chamber 8 and operated by a thermostat responsive to temperature changes in this chamber, thereby preventing flow to the chamber unless the temperature therein is sufiicient for decomposing all the ammonia gas fed to the chamber.

When the thermostatic valve is thus opened, dissociated gas discharges through the outlet 8a, for example, at a rate of about 24 cubic feet per hour, and flows down through the annular space 12 surrounding the catalyst chamber. I have found that it is particularly important to arrange for the hot exit gas to flow past the catalyst chamber in this manner, for heat losses tend to be extremely high in an apparatus of such small size, and by surrounding the catalyst chamber with a flowing blanket of hot gas, the heat losses from the chamber are greatly reduced. I have also found it advantageous to provide from 0.1 inch to 1 inch of insulation 6a between the heating coil and the hot exit gas in order to reduce further the heat losses from the catalyst chamber. With this flow arrangement, I find'that I can produce 24 cubic feet per hour of dissociated gas with a heating element of 200-1000 watts, depending upon the amount of insulation in the jacket 2| and the cover head or block [5.

The dissociated 'gas flows down the annular space [2 and passes over the heat exchange coil '5. where heat is transferred to incoming ammonia gas and the exit gas is thereby cooled. I have found the design of this heat exchanger and its relation to'other features of the apparatus are important. In the decomposition of am monia into nitrogen and hydrogen there is a two to one increase in the number of molecules of gas. As aresult the sensible heat content of dissociated gas, at any temperature level, is greater than that of the-corresponding quantity of ammonia gas from which it was formed. Hence, even with aperfect heat exchanger, itls not possible to produce sufficient exit gas cooling merely by exchange-against incoming ammonia feed. This is one of the great disadvantages of standard ammonia dissociators, which discharge the dissociated gas at'temperatures above 250 F. It is obvious that this is undesirable because this temperature level is high enough to cause severe burns should. theapparatus'be operated by untrained or careless personnel.

changes of temperature in the chamber. The exit gas before it'enters'intofinal'heat exchange relationship with the incoming gas. The incoming gas is the final cooling" medium, inasmuch as it is necessarily at a temperature level lower than that of the surroundings, since it was vaporized by transfer of heat from the surroundings. The drawing shows the novel manner in which I have made this possible. The insulation 2| extends down over the upper part of the heat exchange section, and then ends. In this upper part of the heat exchanger, incoming ammonia gas receives its final preheat, upon which a good part of the thermal efficiency of the apparatus depends. In the heat exchange section below the insulation 2 I, heat is removed from the exit gas in two ways: (a) through the wall of the coil 5 conducting the relatively cold ammonia gas and (b) through the heat conducting outer wall II of the annular space I2. Since the lower part of the member II consists only of metal or other good conductor of heat, it allows the exit gas flowing through the lower part I22) of the heat exchange space to heat air which enters through ports 22a and, by virtue of the fact that it is being heated, circulates upward through air space 23 and outward through exit ports 222). In the very bottom section of the heat exchange space I2b the final cooling of the exit gas is effected largely by exchange with the -cool incoming feed in coil 5, although some heat is still being dissipated to the air in the surrounding space 23. In this manner, it is possible to lower the temperature of the exit gas to well below 250 F., and by good design to below 150 F.

In order to satisfy these requirements and others which are dependent thereon, to be hereinafter described, I find that, for the production of up to 50 cubic feet per hour of dissociated gas, the tubing employed in the heat exchanger coil 5 should be between /64 inch and inch in diameter, preferably between /32 inch and 4 inch in diameter, and that the external surface of the coil should be between 0.1 square foot and 5 square feet, preferably between 0.3 square foot and 1.2 square feet. In order to obtain the maximum benefit of heat transfer rate and to cut down holding volume, for reasons hereinafter described, I have also found that the thickness of the annular space I2--I2b should be as small as practicable and still accommodate the heat exchange coil.

The best results are obtained if the thickness of this annular space exceeds the outer diameter of the coil 5 by no less than 0.01 inch and by no more than 0.2 inch. For good performance, I have found that between 25% and 75% of the sensible heat of the gas leaving the catalyst chamber 8 should be dissipated to the surroundings. In order to produce this result, for an apparatus having a capacity of up to 50 cubic feet per hour of dissociated gas, the lower or uninsulated heat transfer area of the outer wall I I of the annular space I2--I2b should be between 0.1 square foot and 5 square feet, preferably between 0.3 and 1.5 square feet.

The example cited above shows some of the advantages of my invention. However, there are still other advantages relating to safety features which permit the use of the apparatus by chil-- dren and inexperienced persons, and certain aspects of these features will now be described.

The gas flow rate is restricted in four different ways: (1) by the restriction orifice 3 in the outlet of the ammonia container I; (2) by the use of the special union 4 between this container and the converter; (3) by the thermostatic valve 9-9a in the catalyst chamber outlet 8a; and (4) by restriction of the external surface of the ammonia container I to a range of surface area between 0.1 and 3.0 square feet. The latter feature is of importance, because ammonia gas will flow only as rapidly as heat can be supplied by the surroundings to evaporate liquid ammonia to gas. For best performance, I have found that the surface area of the ammonia container should be between 0.2 and 1.5 square feet.

When the container I is disconnected from the converter, unconverted ammonia in the duct 5 and the reaction zone 8 is relatively free to flow out to the atmosphere, especially if this zone is at superatmospheric pressure when the union 4 is disconnected. In the form of my apparatus for domestic use, the internal volume of this zone should be limited to less than 0.1 cubic foot and preferably to less than 0.01 cubic foot, so that the amount of ammonia vapor discharged into an open room is not objectionable and is far removed from the quantity that would have a toxic effect upon humans. I can further reduce the quantity of ammonia gas which can thus escape into the room by installing a back flow check valve in the union section 4b, but I have not found this necessary and have found that by not using it the possibility of creating excessive pressures within the apparatus under certain circumstances is eliminated.

The employment of a narrow, elongated path between the union 4 and the reaction chamber 8, generally slanting in an upward direction, greatly retards the diffusion of air into the apparatus and prevents reoxidation of the catalyst with air during such periods when the container I is disconnected from the converter. This allows the catalyst to be reduced in situ after initial assembly of the apparatus without the necessity of reduction prior to each use, which would, in this small equipment, consume an inordinate length of time and quantity of ammonia. The thermostatic valve also prevents the infiltration of air into the catalyst by backfiow or diffusion through the outlet line.

The sole control mechanism, the electric switch I9, can be equipped with an automatic time outoff, as previously stated, so that it will shut off after a specified period of time if left unattended. This will result in cooling of the catalyst chamber 8 and closing of thermostatic valve l-Ea, thereby halting flow of ammonia and preventing an ex.- cessive quantity of dissociated gas from being discharged into a room.

The apparatus in its domestic form also greatly limits the volume of the outlet section of the system, from the reaction chamber 8 through the annular space I2I2b to the outlet pipe I3. The volume of this space is less than 0.2 cubic foot, and for preferred design less than 0.01 cubic foot. This limits the amount of air which can infiltrate this part of the apparatus while it is idle, to the point where (a) it will not seriously impair the lifting power of the first balloonful of gas and (b) any explosive mixture of hydrogen and air will be so small in quantity that it could not possibly damage personnel or property.

The catalyst or reaction chamber 8 in my apparatus is so designed that it meets the following requirements: (a) sufficient surface for the transfer of heat to decompose the requisite quantity of ammonia; (b) sumcient volume to hold enough catalyst for ammonia decomposition at the desired rate of flow; (0) small enough volume to reduce the retained volume of NH: during non-use to the point where, should the union able nor dangerous. three requirements compatible, I have found that 4 be disconnected, the amount 1 of ammonia dis- Jchargedinto a :room would be neither objection- In order to make these ratio -of the output to the weight of the apparatus, coupled with the fact that the only operating control it requires is a single electric switch, makes the apparatus particularly suited for'nonindustrial uses. Depending upon the desired capacity of the apparatus in the range'of 10'to50 cubic feet of dissociated gas per hour, for example, the converter can be made in a form 3 t 8 inches in diameter, 8 to 30 inches in height, and 3 to 20 pounds in overall weight, so that it can be handled easily by a single person, even a child.

It is to be understood that the details of the apparatus may be varied at will within wide limits. For example, the outer jacket 22 may be permanently sealed so that none of the operating parts can be damaged by inexperienced persons, and it may be provided with additional insulation sufficient to keep the outside of the apparatus cool. Also, a suitable safety device can be added to prevent overheating or the electric heating element ll.

While the apparatus is intended primarily for use by children, for bringing to the home for the first time an apparatus capable of filling floating balloons, I do not intend that it shall be limited to any specific purpose. As an'example, it will also be extremely useful as a means-of producing reducing gas in small laboratories, and providing hydrogen-containing gas for meteorological balloons at remote weather stations. Also, while the features of the invention are particularly suited to small apparatus, for capacities up to 50 cubic feet per hour of dissociated gas, I do not intend that they should be limited or restricted to any particular production rate.

I claim:

Apparatus for dissociating ammonia into a mixture of nitrogen and hydrogen, which comprises a base, a heat insulating core extending upwardly from the base and forming in its upper portion a catalyst chamber, an electric heating 'coil in the core surrounding the chamber and operable to heat the chamber to a temperature at which "ammonia gas therein is dissociated in thepresenc'e of the catalyst, a hollow member'on the base surrounding the core and forming therewith a discharge passage for dissociated ammonia, said passage communicating with an outlet from the upper portion of the chamber, a cover on the upperend of the hollow member for sealing said passage, a conduit leading from the lower portion of said passage below the chamber for discharging dissociated ammonia, a duct leading upwardly through said passage to the lower :part of the catalyst chamber for feeding ammonia gas thereto, a valve in proximity to said chamber outlet and normally blocking the flow of gas from the chamber, a thermostat opera tively connected to the valve and responsive to temperature changes in the chamber, said thermostat acting to open the valve only when the chamber is heated to a temperature sufflcient'to WALTON Hl MARSHALL, JR.

-REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number I Name Date 180,891 Kraushaar Aug. 8, 1876 1,479,110 Skelly Jan. 1, 1924 1,839,413 Sage Jan. 5, 1932 1,915,120 Burke June 20, 1933 1,940,355 Knapp Dec. 19, 1933 1,966,610 Chilowsky July 17, 1934 1,979,187 Bindley Oct. 30, 1934 2,013,652 Hall Sept. 10, 1935 2,104,333 Rosenblad Jan. 4, 1938 2,140,254 Zavka Dec. 13, 1938 2,161,746 Pearson June 6, 1939 2,285,305 Reid June 2, 1942 2,374,639 Miller Apr. 24, 1945 FOREIGN PATENTS Number Country Date 293,007 Great Britain Nov. 8, 1928 OTHER REFERENCES Serial No. 365,412, Picconi (A. P. C.), published April 27, 1943. 

